Owing to increase in information transmission capacity in recent years, higher transmission speed is required for optical transmission systems. At present, research and development for ultra-high speed optical transmission systems with the transmission rate of 100 Gb/s has been advanced. In the ultra-high speed optical transmission system with transmission rate of 100 Gb/s, in order to maintain compatibility with the relay distance in the conventional optical transmission system, it is required to improve SNR (Signal to Noise Ratio) and the resistance to wavelength dispersion and polarization mode dispersion. Therefore, for the ultra-high speed optical transmission system realizing the transmission rate of 100 Gb/s, the method is thought to be predominant which demodulates the polarization multiplexing optical signal by means of the digital coherent system. As for the polarization multiplexing optical signals, for example, it is available to use the optical signals obtained by multiplexing a vertically polarized optical signal and a horizontally polarized optical signal, each of which is modulated according to quadrature-phase-shift-keying (QPSK).
An example of the optical transmission system using such polarization multiplexing optical signals is disclosed in Japanese Patent Application Laid-Open Publication No. 2008-263590. In this related polarization multiplexing optical transmission system, a polarization multiplexing optical transmitter modulates at least any one of the wavelength, the transmission timing, and the intensity of the light as a carrier wave, by using predetermined pilot signal which can be recognized between the polarization multiplexing optical transmitter and the polarization multiplexing optical receiver. The polarization multiplexing optical receiver extracts a component of the predetermined pilot signal from a separated polarization component, and separates the polarization multiplexing signals by controlling their polarization states so that the intensity of this component may become either maximum or minimum.